GABA antagonists applied to the ventral surface of the medulla oblongata block the baroreceptor reflex

Application of gamma-aminobutyric acid (GABA) antagonist drugs (bicuculline, picrotoxinin) to the intermediate region of the ventral surface of the medulla oblongata of gallamine paralyzed, alpha-chloralose anesthetized cats produced a dose-dependent blockade of the baroreceptor reflex. GABA antagonists applied to the rostral and caudal areas did not block the baroreceptor reflex. When strychnine was applied to the ventral medulla, it did not block the baroreceptor reflex. The diffusion of [3H]bicuculline was studied by scintillation counting and autoradiography and found not to penetrate more than 2 mm from the ventral medullary surface. These results suggest that a GABAergic synapse lies within the first 2 mm of the ventral medulla and is involved in baroreceptor modulation of the sympathetic outflow.